Startup system ordering not working? | Bevy | Page 1

Here is an example

use bevy::prelude::*;

fn main() {
    App::new().add_plugins((CharacterPlugin,HatPlugin)).run();
}

struct CharacterPlugin;
impl Plugin for CharacterPlugin {
    fn build(&self, app: &mut App) {
        println!("Building CharacterPlugin");
        app.configure_sets(Startup, CharacterSystemSet::Spawn)
            .add_systems(Startup, spawn_character.in_set(CharacterSystemSet::Spawn));
    }
}

#[derive(SystemSet, Clone, Hash, Debug, PartialEq, Eq)]
pub enum CharacterSystemSet {
    Spawn,
}

#[derive(Component, Default, Reflect)]
struct Character;

fn spawn_character(mut commands: Commands) {
    let id = commands.spawn((SpatialBundle::default(), Character)).id();
    println!("Spawned character with id: {:?}", id)
}

struct HatPlugin;
impl Plugin for HatPlugin {
    fn build(&self, app: &mut App) {
        println!("Building HatPlugin");
        app.configure_sets(
            Startup,
            HatSystemSet::Spawn.after(CharacterSystemSet::Spawn),
        )
        .add_systems(
            Startup,
            spawn_hat.in_set(HatSystemSet::Spawn).after(apply_deferred),
        );
    }
}

#[derive(SystemSet, Clone, Hash, Debug, PartialEq, Eq)]
pub enum HatSystemSet {
    Spawn,
}

#[derive(Component, Default, Reflect)]
struct Hat;

fn spawn_hat(mut commands: Commands, all: Query<Entity>, characters: Query<Entity, With<Character>>) {
    println!("Found {:?} entities", all.iter().count());
    for entity in all.iter() {
        println!("Entity: {:?}", entity);
    }
    let found = characters.single();
    println!("Found character: {:?}", found);
    commands.entity(found).with_children(|parent| {
        parent.spawn(Hat);
    });
}

Running `target\debug\examples\system_ordering.exe`
Building CharacterPlugin
Building HatPlugin
Spawned character with id: 0v0
Found 0 entities
thread 'TaskPool (8)' panicked at examples\system_ordering.rs:57:28:
called `Result::unwrap()` on an `Err` value: NoEntities("bevy_ecs::query::state::QueryState<bevy_ecs::entity::Entity, bevy_ecs::query::filter::With<system_ordering::Character>>")
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Encountered a panic in system `system_ordering::spawn_hat`!
Encountered a panic in system `bevy_app::main_schedule::Main::run_main`!
error: process didn't exit successfully: `target\debug\examples\system_ordering.exe` (exit code: 101)

Why is the hat system not seeing the other one?

found the problem

// BAD
my_system.after(apply_deferred)
// GOOD
(apply_deferred, my_system).chain()

#Startup system ordering not working?